This invention relates to optical devices, and in particular to a polarization independent optical circulator.
With the growth of lightwave communications, the need for a variety of new optical devices has emerged. For example, multi-port optical circulators may be used to couple light between a transceiver (combination transmitter and receiver) and a bi-directional optical fiber. Early proposals for such circulators typically included a pair of prisms rotated 45 degrees with respect to each other about their optical axes and a Faraday rotator therebetween. The prisms determined the light path according to its direction of polarization and the rotator caused a 45 degree rotation of polarization so that light from one port would be incident on only one other port. (See, e.g., Electronics Letters, Vol. 14, page 816 (1978).) A basic problem with such circulators was that they were polarization dependent, i.e., they relied on an incident beam having linearly polarized light in a particular direction. Since optical fibers cannot generally preserve optical polarization, it was recognized that an efficient optical circulator should preferably be polarization independent.
Consequently, a variety of polarization independent circulators were proposed. Such devices basically include some means for splitting an incoming beam from one port into two beams having orthogonal polarizations, some means for recombining the two beams at another port, and means for nonreciprocal rotation of the polarization directions of the beams by odd or even multiples of 90 degrees depending upon the direction of the beams. This rotation means typically includes a Faraday rotator in combination with a half-wave plate. The means for splitting and recombining the beams and directing them through the rotator have generally included combinations of prisms which may be difficult to manufacture and align in the final structure and could produce significant insertion losses. (See, e.g., U.S. Pat. No. 4,272,159 issued to Matsumoto; Electronics Letters, Vol. 15, No. 25, pp. 830-831 (December 1979); and Applied Optics, Vol. 20, No. 15, pp. 2683-2687.).
It is known in the art of making optical isolators to utilize birefringent plates, rather than prisms, to separate and recombine beams having orthogonal polarizations. (See, e.g., U.S. Pat. No. 4,178,073 issued to Uchida et al and U.S. Pat. No. 4,239,329 issued to Matsumoto.) Such plates are easier to fabricate and align in the final device structure. However, their use in multi-port circulators has not been suggested previously to the best of my knowledge.
It is therefore an object of the invention to provide a multi-port optical circulator which is easy to fabricate and assemble and does not produce significant insertion losses.